When you open such a file, a window with a progress bar shows that the file is converting. To stop converting, just close the window.

Media files that use many of these older formats will not be compatible with future macOS releases, so you should save the converted file before closing it. Conversion doesn't modify or replace the original file.

To preserve the quality of the original file, the converted file might be larger than the original.

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QuickTime Player uses Apple ProRes to convert legacy formats used in video workflows, such as those encoded with the Animation codec. It uses H.264 to convert other older or third-party media formats.